The present invention relates generally to reading and decoding information symbols, and more specifically to a laser scanning system and method for reading information symbols such as optical mark recognition symbols.
Machine readable information labels or codes, such as bar codes, are ubiquitous in today""s world. Bar codes are utilized for myriad different purposes, being affixed to many consumer products to identify the cost of the products, and being utilized in industry to identify components during manufacture and items stored in inventory. A bar code consists of a series of bars and spaces of varying widths formed according to a set of rules to thereby encode data, as will be understood by those skilled in the art. In addition to bar codes, other types of machine readable codes are utilized in particular industries to encode machine readable data. One such code is known as Optical Mark Recognition (xe2x80x9cOMRxe2x80x9d) symbology, which is utilized in the document handling industry to encode data in OMR symbols that are affixed to documents. The document handling industry includes such companies as Xerox, Hewlett-Packard, and Pitney Bowes, which utilize OMR symbols to encode geographic regions or zip codes for use in sorting the corresponding documents.
FIG. 1 illustrates a conventional OMR scanning system 10 including an OMR symbol 11 and an optical sensor 38 for reading the OMR symbol, as will be explained in more detail below. The OMR symbol 11 includes a plurality of cells 12-26 arranged as shown, each cell 12-26 containing a single bit of binary data represented by either a corresponding mark or bar in the cell, or a space in the cell. In the OMR symbol 11, the cells 12, 16, 20, and 24 include bars 28, 30, 32, and 34, respectively, while cells 14, 18, 22, and 26 include spaces (i.e., no bar). Typically, each of the bars in the cells 12, 16, 20, and 24 represents a binary 1, and each of the spaces in the cells 14, 18, 22, in 26 represents a binary 0. Although the OMR symbol 11 is shown having the eight cells 12-26, the number of cells in an OMR symbol may vary, with there typically being between 8-32 cells in a symbol.
In a typical application, the OMR symbol 11 is attached to an object, such as a letter or package, and contains bars and spaces in the cells 12-26 to encode the desired data. The object is typically placed on a conveyor belt (not shown) and is thus moving at a velocity VO relative to the optical sensor 38. The optical sensor 38 applies incident optical energy 40 to each cell 12-26 of the OMR symbol 11 as that cell passes by the sensor. As each cell 12-26 passes by the optical sensor 38 moving at the velocity VO, the sensor 38 senses optical energy reflected from the cell 12-26 to thereby detect the presence of a bar or space in each of the cells. In FIG. 1, the ONR symbol 11 a shown positioned with the cell 18 being illuminated by the optical energy 40 from the sensor 38. As understood by those skilled in the art, the presence of a bar in a cell results in optical energy being absorbed when incident optical energy is applied to that cell, while a space (no bar) results in optical energy being reflected when incident optical energy is applied to the cell. Thus, in FIG. 1, the optical sensor 38 detects optical energy being reflected from the cell 18, indicating that the cell 18 cell contains a space. From the detected bars and spaces in each of the cells 12-26, the optical sensor 38 generates binary data corresponding to the decoded OMR symbol 11, each bit in the binary data corresponding to one of the cells 12-26 in the OMR symbol.
The optical sensor 38 is typically an LED sensor or a fixed-beam laser type device, as will be understood by those skilled in the art. Such devices may have difficulties dealing with so-called xe2x80x9cpaper flutterxe2x80x9d of the OMR symbol 11 which occurs when the document to which the OMR symbol is affixed moves towards or away from the LED sensor or fixed-beam laser. Moreover, poor contrast between bars in the OMR symbol 11 and the surface to which the symbol is affixed also presents difficulties for the LED and fixed-beam laser type devices. In addition, LED and fixed-beam laser type devices cannot read OMR symbols in xe2x80x9cladderxe2x80x9d orientations. As will be understood by those skilled in the art, information symbols are typically read in either a ladder or picket fence orientation. A ladder orientation results when the lengths of the bars in the OMR symbol are parallel to the symbols direction of travel, and a xe2x80x9cpicket fencexe2x80x9d orientation occurs when the lengths of the bars in the OMR symbol are perpendicular to the direction of travel (as depicted in FIG. 1).
In contrast to the LED and fixed-beam laser type devices, laser scanners are less affected by poor symbol contrast or paper flutter, and can read OMR symbols in either picket fence or ladder orientations. A laser scanner directs a laser spot across a scan window containing a bar code label. As the laser spot travels across the scan window SW, the laser scanner detects reflected optical energy from a bar code label contained within the scan window and utilizes this reflected optical energy to decode the bar code label. Due to the physical construction of the laser scanner, the velocity of the laser spot, which is known as the xe2x80x9cspotxe2x80x9d velocity, varies as the laser spot travels across the scan window. During normal operation of a laser scanner in reading bar code labels, the varying spot velocity does not result in difficulties in reading the labels. This is true because the predictable characteristics of bar code labels that allow for compensation of the varying spot velocity during decoding, When a laser scanner is utilized to read OMR symbols moving relative to the laser scanner, however, difficulties arise in reliably reading such labels due to the varying spot velocity of the laser spot. The operation and characteristics of laser scanners will be understood by those skilled in the art, and thus a detailed description of such operation and characteristics has been omitted for the sake of brevity.
Referring to FIG. 1, the line 36 represents the path of a laser spot from a laser scanner during scanning of the OMR symbol 11. The spot velocity of the laser is designated Vs, and the length of the line 36 from left to right corresponds to the scan window SW of the laser scanner. The spot velocity VS varies across the scan window SW due to the planar surface of the OMR symbol 11 truncating an arced path of the laser beam, as will be understood by those skilled in the art. If it was attempted to read the OMR symbol 11 with a laser scanner, the varying spot velocity VS and velocity VO of the symbol 11 result in difficulties in decoding the symbol. For example, the spot velocity VS is slower towards the center of the scan window SW. Thus, the slower spot velocity VS combined with the symbol velocity VO result in the laser beam illuminating the interior cells 18 and 20 for a longer duration than the cells 12-16 and 22-26. In decoding an OMR symbol, a valid bar is typically detected by reflected optical energy for at least a predetermined time. Due to the variable spot velocity VS, this predetermined time will be longer for the interior cells 18, 20 than for the exterior cells 12-16 and 22-26, thereby making the detection and decoding of the OMR symbol 10 difficult.
The variable spot velocity VS in combination with the variable or xe2x80x9cfree-formxe2x80x9d nature of OMR symbols has precluded reliable decoding of the symbols using laser scanners. OMR symbols are free-form in that only the first cell in an OMR symbol must contain a bar, known as a xe2x80x9cgatexe2x80x9d bar, and all other cells may contain either bars or spaces. In FIG. 1, the bar 28 in cell 12 may be the gate bar, and all other cells 14-26 may contain any combination of bars and spaces. This free-form format of OMR symbols makes it difficult to detect where a first OMR symbol ends and where a second symbol begins, and also makes it difficult to detect spaces and bars within each symbol, as will be appreciated by those skilled in the art.
There is a need for a laser scanning system that reliably senses and decodes free-form information symbols such as OMR symbols.
A scanning system and method reads information symbols, such as optical mark recognition (xe2x80x9cOMRxe2x80x9d) symbols. Each symbol includes a number of cells, with each cell containing a bar or a space. According to one aspect of the present invention, a symbol scanning device includes an optical transmitter that generates a scanning beam that scans each symbol when the symbol is within a scan window. An optical detector in the scanner is positioned to receive optical energy reflected from each symbol responsive to the scanning beam. The optical detector generates a detection signal responsive to the received optical energy. A processing circuit is coupled to the optical transmitter and the optical receiver. The optical transmitter scans a symbol within the scan window, and the processing circuit thereafter processes the corresponding detection signal from the optical detector to read the scanned symbol. The optical transmitter may be a scanning laser.